Polytetrafluoroethylene (alternately referred to herein as PTFE) has a low friction coefficient and excellent heat and chemical resistance. However, fillers often need to be used with PTFE to prepare a PTFE composition with adequate wear resistance or creep resistance for a given use. Fillers for PTFE known in the art include inorganic fillers such as glass fibers, carbon fibers, graphite, molybdenum disulfide, and bronze powder, and organic fillers such as aromatic polyester, polyimide, polyphenylene sulfide, and aromatic polyamides. PTFE molding powder and filler compositions are not melt processible but can be molded into sliding members or seal members of commercial utility by processes such as compression molding.
However, the number of commercial manufacturing processes for sliding members such as seal rings is large, and the commercial mass production of such sliding members from PTFE compositions is not easy due to the fact that PTFE is not melt processible. As a results, processes for the manufacture of PTFE sliding members often result in significant waste and accompanying undesirable costs.
In order to overcome such productivity problems, the literature reports attempts to mix the aforementioned fillers with melt processible fluororesins such as tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (alternately referred to herein as PFA), tetrafluoroethylene-hexafluoropropylene copolymer (alternately referred to herein as FEP), and tetrafluoroethylene-ethylene copolymer (alternately referred to herein as ETFE), each of which are melt processible and have excellent moldability, though their friction characteristic is inferior to that of PTFE, and make the moldability and the frictional wear characteristic compatible to obtain sliding members or seal members. For example, a composition in which graphite, polyphenylene sulfide or aromatic polyamide fibers, etc., are mixed into a melt processible fluororesin, which can be melt-processed by injection molding, etc., are proposed in the literature.
In addition, lightness, miniaturization and environmental measures have recently been advanced in various kinds of hydraulic apparatuses or sliding machines, increasing opportunities to use lightweight soft metals such as aluminum alloys.
In the case where the sliding counterpart member is a lightweight soft metal such as aluminum alloy, since the counterpart metallic material is apt to be worn and damaged by a sliding member or seal member molded of a conventional resin composition containing inorganic fillers, members to which organic fillers (particles or fibers) such as polyimide resin or wholly aromatic polyamide resin, which have high strength and are softer than inorganic fillers, are added have frequently been used. See for example such disclosures in: Japanese Kokai Patent Application No. Hei 2 [1990]-163147: Japanese Kokai Patent Application No. Hei 6 [1994]-122887; and Japanese Kokai Patent Application No. 2006-225433.
As mentioned above, to overcome the productivity problem due to the non-melt processibility of PTFE, attempts have been made to obtain sliding members or seal members from melt processible fluororesins such as PFA, FEP, or ETFE with excellent moldability, although the friction characteristic of these resins is inferior to that of PTFE. However, since the melt processible fluororesins such as PFA are inferior in friction characteristic compared with PTFE, use of a filler is essential. However, since the addition of the filler could also deteriorate the moldability of the melt processible fluororesin, it was difficult to develop a composition that raises the frictional wear characteristic while maintaining good moldability as an advantage of the melt processible fluororesins and meets performances required for sliding members. In addition, as fillers that do not damage the counterpart member, the utilization of organic fillers has recently been reviewed. However, thermoplastic resin fillers were melted during kneading with a fluororesin, and as a result being unable to exert an effect as fillers. Moreover, in case fibrous fillers were used, sufficient melt-mixing was impossible because of the entanglement of the additive fillers, causing segregation, thus being unable to exert a stable frictional wear characteristic.